(1) I believe "Abyssal one" is an external designator, much like "Claymore" -- it's not a different species. It's just some people who were really strong before, then they awakened and now they're also really strong. Sort of like the designation "Sannin" in Naruto.

(2) Awakened beings -- even the weak ones -- are substantially stronger than any ordinary yoma. If you want to think in terms of Teresa and Clare and other claymores and their yoma, consider that while Clare is only what you're calling a quarter yoma, she's also a quarter teresa -- who was also way stronger than any yoma. If you consider her first near-awakening as a foray into tapping the power of Teresa and her yoma parts, then her subsequent experiences involved learning to better tap those constituents.

(3) While you can compare things like "size of yoki" on a direct scale, fighting is a combination of a lot of factors. Clare would have been last on yoki, but she's proven time and again that last on yoki doesn't mean worst in a fight.